Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

From: Kees Cook <keescook@chromium.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Mateusz Guzik <mjguzik@gmail.com>,
	Eric Biggers <ebiggers@google.com>,
	Alexander Potapenko <glider@google.com>,
	Christian Brauner <brauner@kernel.org>,
	serge@hallyn.com, paul@paul-moore.com,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible
Date: Thu, 2 Mar 2023 11:54:03 -0800	[thread overview]
Message-ID: <6400fedb.170a0220.ece29.04b8@mx.google.com> (raw)
In-Reply-To: <ZAD21ZEiB2V9Ttto@ZenIV>

On Thu, Mar 02, 2023 at 07:19:49PM +0000, Al Viro wrote:
> On Thu, Mar 02, 2023 at 11:10:03AM -0800, Linus Torvalds wrote:
> > On Thu, Mar 2, 2023 at 11:03 AM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > It might be best if we actually exposed it as a SLAB_SKIP_ZERO thing,
> > > just to make it possible to say - exactly in situations like this -
> > > that this particular slab cache has no advantage from pre-zeroing.
> > 
> > Actually, maybe it's just as well to keep it per-allocation, and just
> > special-case getname_flags() itself.
> > 
> > We could replace the __getname() there with just a
> > 
> >         kmem_cache_alloc(names_cachep, GFP_KERNEL | __GFP_SKIP_ZERO);
> > 
> > we're going to overwrite the beginning of the buffer with the path we
> > copy from user space, and then we'd have to make people comfortable
> > with the fact that even with zero initialization hardening on, the
> > space after the filename wouldn't be initialized...
> 
> ACK; same in getname_kernel() and sys_getcwd(), at the very least.

FWIW, much earlier analysis suggested opting out these kmem caches:

	buffer_head
	names_cache
	mm_struct
	anon_vma
	skbuff_head_cache
	skbuff_fclone_cache

Alexander's analysis more recently[2] of skbuff went a bit further,
I think, and allowed opt-out for non-kmem cache page allocations too.

-Kees

[1] https://lore.kernel.org/all/20190514143537.10435-5-glider@google.com/

-- 
Kees Cook